In conventional recirculating air heating apparatus and other such process apparatus, it is common to use transverse or cross-web air jets to impinge the air against a surface or material being treated. Heat transfer rates attainable by impingement of air jets is greater than normally associated with gas phase heat transfer. This leads to compact, low cost equipment and more precise control of processing conditions.
Practical application of air jets will normally employ an array of the transverse or cross-web jets. Generally speaking, maximum heat transfer rates are attained with a large number of smaller jets. However, minimum jet dimensions are usually limited by practical considerations such as the increased tendency of smaller nozzles to plug. In this regard, lint or fines accumulation, particularly at places in the air flow system having directional vanes or sharp breaks, can result in uneven or broken air flow at the nozzles. Although filters may be used extensively in recirculating type dryers, the filters are incapable of removing very fine lint particles or fines which tend to build up and cause accumulations at the areas of greatest change in the direction of flow in the pressurized recirculation system.
Also of critical importance, particularly for thermally sensitive operations where uniformity and control of localized rates is vital to optimum performance, is the nozzle arrangement and design. Conventionally it has been the practice to provide deflectors or directional vanes interiorly of the nozzle structure to direct the air flow so that it strikes the web being heat treated substantially perpendicular thereto. This is desirable and necessary for proper heat treating or material processing, eliminating unbalanced, transverse impact or sensitive surfaces. However, the use of directional vanes can result in lint or fines accumulation in the nozzles and again uneven, transverse or cross-web air flow. This in turn can cause stripping of the web and/or uneven heat treating or processing in addition to making the apparatus less efficient overall.
In prior U.S. Pat. No. 3,429,057, assigned to assignee of the present application, there was discussed a novel nozzle arrangement for dryers and other air treating apparatus which consisted of at least one pair of side-by-side ducts positioned across the path of travel of the material being treated. Each duct was provided with a continuous, longitudinally extending air nozzle which projected outwardly from the duct towards the material being treated. The nozzles were spaced from the material so that the gaseous medium flowing from one nozzle converged and merged with the gaseous medium flowing from the other nozzle forming a common stream at least at the path of travel of the material. One of the ducts was fed by treating medium from one side of the material being treated and the other duct was fed by treating medium from the opposite side of the material being treated. Thus the main directions of flow of the treating medium in the two ducts were opposite one another with the result that the flows from the nozzles had opposite longitudinal velocity components in addition to vertical velocity components. The longitudinal velocity components of the two streams tended to cancel each other, and the high velocity vertical components of one stream reinforced the low velocity vertical components of the other at any given location along the nozzle, and vice versa such that the common stream produced by the combined flows was uniform along its length and essentially vertical in direction. This also resulted in a swirling motion in the common stream which impinged against the material being treated increasing the efficiency of treatment. Because the nozzle openings were continuous and unbroken, this insured an even, unbroken stream of fluent medium against the material being treated.
Whereas the invention of prior U.S. Pat. No. 3,429,057 constituted a substantial advance in the art, particularly for the plastic film industry where cross-web uniformity of treating medium is critical, it was found that the protruding nozzle lips of the nozzle arrangement were vulnerable to entanglement with film if or when a break in the web occurred. It is a simple matter to simply eliminate the protruding nozzle lips and employ a sharp-edged orifice, but it was found that such a modification or change would create a very high static pressure drop in the nozzle. Since the fan horsepower for recirculation heat treating apparatus is directly proportional to static pressure, the very high static drop created a fan horsepower consumption which was far in excess of that desired for commercial recirculation heat treating apparatus.